1. Field of the Invention
The present invention relates to recording readback systems for data recovery, and particularly to optical disk file data recovery systems.
2. Description of the Related Art
Conventional large capacity information storage systems utilize magnetic or optical disk drive systems to store large amounts of information while still providing good access times. One important aspect of a disk drive unit is the readback system that processes the information read from the disks. Such a readback system often includes a number of electrical circuits. A well-designed readback system should provide accurate, reliable, data readout at a high speed and still be cost effective. As for most electrical circuits, cost reduction can be achieved by designing readback circuits for reduced power consumption and reduced chip space.
Conventional magnetic disk drives, including hard disk units and floppy disk units, have many advantages. Magnetic disk drive units can both read and write. The storage capabilities of hard drive units have been steadily increasing, such that one gigabyte disk drive units with very fast access times in a standard computer form factor are becoming common. On the other hand, floppy disk drives are substantially slower and have much smaller capacities, but still remain popular primarily due to convenience. Floppy disk drives allow disks to be inserted and removed for use by other users and in other computers.
Optical disk units combine the benefits of large storage capacity, fast access times, and removability. In an optical disk drive unit, a laser beam is directed to an optical disk that has data stored on it in a predetermined format. The reflected light is detected by one or more photodetectors that provide an electrical signal which can then be processed by a readback circuit. Optical disk drive units can provide significantly greater storage than magnetic disks and also provide significantly faster access times. Several types of disk drive units are currently available: CD-ROM (Compact Disk-Read Only Memory) units, WORM (Write-Once, Read-Many) units, and MO (Magnetic-Optical) units.
For reading only, CD-ROM (Compact Disk-Read Only Memory) units are available at a price that makes them attractive to many computer users. CD-ROM disks can be manufactured inexpensively, they are portable, and have large storage capacity. However, CD-ROM disks are not writeable because the digital data is stored in a plurality of permanent indentations on the disk.
WORM units provide write capability for one time only. To write data, WORM units direct a laser beam to create grooves in data sectors of special disks. Once written, a WORM disk cannot be easily erased and re-written, and therefore WORM units have been used mostly for archival purposes.
MO units, on the other hand, provide unlimited capability to both read and write on an MO disk. Each track on an MO disk includes MO data sectors and relatively small ROM sectors for identifying the MO sector that follows. The MO data sectors are designed to allow writing a series of magnetic marks. To read the magnetically-marked data sectors, the laser beam is directed to each of the marks which changes the beam's polarization upon its reflection from the magnetically-marked area on the disk. The polarization change in the reflected beam is detected by two photodetectors. Although MO units provide large amounts of data storage and very fast data access in a removable disk, MO units have not yet become extremely popular for read/write uses, at least in part due to their large cost in comparison to magnetic hard drive units.
One standard data format for optical disks is PPM (Pulse Position Modulation). In a PPM format, data is stored in the form of a series of like-size spots, and the information content is in the distance between the like-size spots. Up until now, PPM has also been conventional for MO units. However, PWM (Pulse Width Modulation) recording, which has been used for CD-ROM units, is now being applied to MO uses. In a PWM recording, information is stored in a series of variable-sized "spots" and the information content is in the distance between the transitions, or "edges", and therefore the spot size becomes critical. An example of PWM's application to MO units is disclosed in U.S. Pat. No. 5,204,848 to Cardero et al, entitled "Adjusting Amplitude Detection Threshold by Feeding Back Timing-Data Phase Errors". PWM recording can advantageously increase storage densities by 50% or more in the same disk area. However, this new technology has created new problems. One significant problem is that the readback path for PWM-formatted data cannot have a high-pass filter with a high (e.g. 300-500 KHZ) threshold. This problem, in turn creates difficulties in correcting baseline variations.
Baseline variations regularly occur in the signal read directly from the disk, and are particularly troublesome in optical readback systems. A "baseline" is a reference value that approximates the level of an electrical readback signal between the highest and lowest excursions of that signal. At least two types of baseline variations are significant: rapidly occurring (transient) baseline variations which occur normally during operation and slow baseline variations caused by voltage decay of the coupling capacitors, which causes the baseline to drop gradually. The gradual decay is at a rate sometimes called the "slew rate".
Transient baseline variations can be caused during normal operation by switching functions within the recording system. In optical recording systems, these transients are especially severe and prevalent. For example, transients may be caused by such events as a change in data types from the read only memory (ROM) header to the magneto-optical (MO) data region. Another cause of transients is a change in laser light level when changing from write, erase, or standby illumination levels to read illumination levels. Other transients occur when laser illumination passes from an area of the disk that has been erased to an area of the disk that has data. In order to provide reliable data detection, the amplitude and length of these baseline variations should be reduced as much as possible.
In readback systems, transient baseline variations have usually been handled by high pass filters including AC coupling capacitors. In order to handle severe baseline variations, a high pass filter could be designed with a high cutoff frequency, such as 500 kilohertz, that allows the circuit to compensate quickly for transients. Such a high pass filter is commonly used for PPM readback systems. However, as mentioned above, a high cutoff frequency in the range of 300-500 KHz is inappropriate for PWM systems.
As mentioned above, a high pass filter with a high cutoff frequency can adequately filter out transients in conventional PPM systems. However, the faster and more effective PWM systems such as disclosed in U.S. Pat. No. 5,204,848, issued to Cardero, et al., and assigned to the assignee of the present invention, can have problems, such as false lockups, if the high pass filter has too high a cutoff frequency. Therefore, effective operation of PWM systems requires that the high pass filter should have a relatively low pass cutoff frequency such as 2 KHz. Unfortunately, a lower cutoff frequency would allow a significant portion of the transient baseline variation to progress through the entire data recovery process, disadvantageously requiring additional dynamic range to supply the necessary increased accuracy. To provide a high dynamic range, very expensive electrical components may be necessary to supply the required number of bits of resolution. One such expensive component is an ADC (Analog-to-Digital Converter). A high dynamic range could require, for example, a high speed 10-bit ADC, whereas a lower dynamic range could require only a 7-bit ADC. It would be an advantage to provide a system that prevents transients from progressing through the entire data recovery process, and thereby significantly reduces the impact of transient baseline variations on the dynamic range requirements of the system.
The second type of baseline variation is a gradual drop in the baseline caused by the slow decay of the coupling capacitors while recovering from a baseline variation. This gradual drop occurs at a rate sometimes termed the "slew rate". Typical baseline restoration circuits for recording systems include a diode that prevents a signal from having so much offset that the signal is clipped. Such circuits do not provide precision baseline restoration. It would be an advantage to provide a system that compensates for the slew rate of the coupling capacitors and thereby further reduces the dynamic range requirements of the system.
In summary, it would be an advantage to provide a system that substantially reduces the dynamic range requirements of the readback circuit, by responding quickly to transient baseline variations. It would be a further advantage if the system follows the slew rate, so that, throughout, the signal from the readback circuit appears to follow an approximately flat baseline.